Enabling Circuit for Avoiding Negative Voltage Transients

ABSTRACT

An enabling circuit for enabling a DC-DC converter having a capacitor coupled to an output terminal of the DC-DC converter to be controlled by a control signal. The enabling circuit may include a comparison circuit configured to compare a feedback signal representative of a charge on the capacitor with a signal representative of a reference charge and to provide an output to in response to the comparison to enable the DC-DC converter to be controlled by the control signal if the charge on the capacitor is less than the reference charge. The enabling circuit may also include a discharge path configured to discharge the charge on the capacitor if the charge is greater than the reference charge. A related system and method are also provided.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of prior application Ser. No.11/142,505 filed Jun. 1, 2005, now U.S. Pat. No. 7,112,943, which itselfis a continuation of prior application Ser. No. 10/738,058 filed Dec.17, 2003, now U.S. Pat. No. 6,906,497, which itself is a continuation ofprior application Ser. No. 10/176,141, filed Jun. 20, 2002, now U.S.Pat. No. 6,756,769, the teachings of which are incorporated herein byreference.

FIELD OF THE INVENTION

This invention relates to an enabling circuit for avoiding negativevoltage transients from an associated regulating circuit, and moreparticularly to such an enabling circuit for enabling a synchronousrectifier converter to switch from a first state to second state if thecharge on an energy storage element of the synchronous rectifierconverter is less than a reference charge.

BACKGROUND OF THE INVENTION

A variety of circuits have energy storage element such as capacitors,inductors, and transformers that transfer energy from an input to anoutput of such circuits. If such energy storage elements are notproperly discharged in some instances, unwanted power disturbances,e.g., negative voltage transients, may occur in the output signalcausing damage to nearby sensitive components.

For instance, such a regulating circuit may be a DC-DC converter. DC-DCconverters generally accept a DC input at one voltage level and convertit to a DC output at a higher or lower voltage level. Such DC-DCconverters may be used in a wide variety of electronic devices inconjunction with a variety of systems. One such system may be used toprovide a battery charging function for portable electronic devices suchas laptop computers, cell phones, pagers, personal digital assistants,and the like.

One type of DC-DC converter is a synchronous rectifier converter (SRC).An SRC does not use any Schottky diodes, but rather uses transistorsreferred to as “synchronous rectifiers.” Such transistors may be avariety of transistors such as MOS or MOSFET transistors. An SRC mayalso have a variety of internal components that typically include anenergy storage element, e.g., a capacitor, inductor, or transformer,with one or more transistors controlled by various control techniques,e.g., pulse width modulation where the switch frequency is constant andthe duty cycle varies with the load.

When an SRC is used in conjunction with a battery power managementsystem, the SRC may accept an input voltage from a number of differentpower sources and convert it to an appropriate output voltage to, amongother things, provide an appropriate charging current to an associatedrechargeable battery. In such a battery power management system, thereis typically an associated controller used to control the batterycharging process. Such controller may be an integrated circuit (IC)having a plurality of input terminals or pins, some of which areconnected to the output of the SRC. For instance, two such terminals maybe coupled to either side of a sense resistor. The sense resistor may bein series with the output of the SRC such that it provides a signalrepresentative of the charging current provided at the output of theSRC.

If a soft start occurs when the energy storage element, e.g., acapacitor, of the SRC is charged at a significant value, e.g., overseveral volts, negative voltage transients may appear on either terminalof the sense resistor potentially causing catastrophic failure of theassociated controller IC. Accordingly, there is a need for an enablingcircuit and method that overcomes the above deficiencies in the priorart and is capable of avoiding negative voltage transients from anassociated regulating circuit by enabling the regulating circuit onlywhen the charge on the energy storage element is below a referencecharge.

BRIEF SUMMARY OF THE INVENTION

According to one aspect of the invention, there is provided an enablingcircuit for enabling a DC-DC converter having a capacitor coupled to anoutput terminal of the DC-DC converter to be controlled by a controlsignal. The enabling circuit may include a comparison circuit configuredto compare a feedback signal representative of a charge on the capacitorwith a signal representative of a reference charge and to provide anoutput to in response to the comparison to enable the DC-DC converter tobe controlled by the control signal if the charge on the capacitor isless than the reference charge. The enabling circuit may also include adischarge path configured to discharge the charge on the capacitor ifthe charge is greater than the reference charge.

According to another aspect of the invention, there is provided asystem. The system may include a DC-DC converter configured to accept aninput power level from a power source and provide a regulated outputpower level to a load. The DC-DC converter may have a capacitor coupledto an output terminal of the DC-DC converter. The system may furtherinclude an enabling circuit for enabling the DC-DC converter to becontrolled by a control signal. The enabling circuit may include acomparison circuit configured to compare a feedback signalrepresentative of a charge on the capacitor with a signal representativeof a reference charge and to provide an output to in response to thecomparison to enable the DC-DC converter to be controlled by the controlsignal if the charge on the capacitor is less than the reference charge.The enabling circuit may also include a discharge path configured todischarge the charge on the capacitor if the charge is greater than thereference charge.

According to yet another aspect of the invention there is provided amethod. The method may include monitoring a charge on a capacitor of aDC-DC converter coupled to an output terminal of said DC-DC converter;preventing the DC-DC converter from being controlled by a control signalif the charge is greater than the reference charge; and enabling theDC-DC converter to be controlled by the control signal if the charge isless than the reference charge.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, together with otherobjects, features and advantages, reference should be made to thefollowing detailed description which should be read in conjunction withthe following figures wherein like numerals represent like parts:

FIG. 1 is a block diagram of a system including an enabling circuitconsistent with the present invention for enabling an associatedregulating circuit to switch from a first state to a second state;

FIG. 2 is a block diagram of an exemplary enabling circuit consistentwith the invention for enabling an associated synchronous rectifierconverter to switch from one state to another state;

FIG. 2A is a circuit diagram illustrating an exemplary embodiment of theenabling circuit of FIG. 2; and

FIG. 3 is a block diagram of a battery management system utilizing theenabling circuit of FIG. 2.

DETAILED DESCRIPTION

Turning to FIG. 1, an exemplary system 100 including an enabling circuit104 and an associated regulating circuit 102 is illustrated. Theregulating circuit 102 may be any variety of circuits, e.g., asynchronous rectifier converter, containing an energy storage element106, e.g., a capacitor. In general, the enabling circuit 104 monitorsthe charge on the energy storage element 106 and enables the regulatingcircuit 102 to switch from a first state to a second state when thecharge on the energy storage element is below a reference charge. Thefirst state may be any variety of states such as a power off state, andthe second state may also be any variety of states such as an operatingstate where the regulating circuit 102 is controlled by a particularcontrol technique. The reference charge should be chosen based on theparticular system and sensitivity of associated components. In oneembodiment, the reference charge may be 3.0 volts.

The enabling circuit 104 has one input terminal 107 configured to accepta signal, Vdsch, representative of an acceptable reference charge levelfor the energy storage element 106. The enabling circuit 104 may haveanother input terminal 109 configured to accept a logic control signalchginh. Such logic control signal, chginh, has a predetermined state,e.g., low state, when at least one non energy storage element conditionrelated to operation of the regulating circuit 102 is satisfied. Such acondition or conditions may be any variety of other conditions known tothose skilled in the art unrelated to the charge on the energy storageelement 106. For instance, one condition may be the proper coupling ofan input power source at a proper power level to the input of theregulating circuit 102.

The enabling circuit 104 accepts a feedback signal from the regulatingcircuit 102 along path 119. Such feedback signal is representative ofthe charge on the energy storage element 106. The enabling circuitcompares the charge on energy storage element 106 with a referencecharge and may output an enabling signal if the charge level is lessthan the reference charge level. The enabling circuit 104 may alsoinclude various discharging means as further described in reference toFIG. 2 in order to discharge the energy storage element 106 below thereference charge level should the charge be greater than the referencecharge level.

In one embodiment, once the charge on the energy storage element 106 isbelow the reference charge, the enabling circuit 104 sends an enablingsignal along path 115 to the controller 120. The controller 120 isresponsive to the enabling signal to then enable the regulating circuitto switch from a first state, e.g., a non-operating state or apredetermined suitable operating state, to a second state, e.g., anotheroperating state. As such, an enabling circuit 104 consistent with theinvention may advantageously delay operation of the regulating circuit102 in the second state while maintaining operation of the regulatingcircuit 102 in the first state until the energy storage element 106 isdischarged below a reference charge.

In another embodiment, the enabling circuit 104 may not send an enablingsignal to the controller 120 until both the energy storage element 106is discharged below the reference charge level and a logic controlsignal, e.g., signal chginh, input to the enabling circuit is at apredetermined state. Such predetermined state is representative of atleast one satisfactory non energy storage element condition. Suchpredetermined state may also be representative of a satisfactorycondition for all other non energy storage element conditions. In thiscase, an enabling circuit 104 consistent with the invention delaysswitching the regulating circuit 102 to operation in the second statewhile maintaining operation of the regulating circuit 102 in the firststate, e.g., a non-operating state or a predetermined suitable operatingstate, until the energy storage element 106 is discharged below apredetermined charge level and a signal representing at least onesatisfactory non energy storage element condition related to operationof the regulating circuit is satisfied.

Turning to FIG. 2, one exemplary system 200 having an enabling circuit204 consistent with the invention and a synchronous rectifier converter(SRC) 202 is illustrated. The SRC accepts an input voltage signal atinput terminal 226 and provides an output voltage signal at outputterminal 211. The SRC may include an inductor 208, a capacitor 206 andswitches K1, K2. Switches K1, K2 may be any type of transistors and forsimplicity are drawn to represent MOS type transistors. Such switchesK1, K2 may be controlled by a variety of control techniques such aspulse width modulation (PWM) where the switch frequency is constant andthe duty cycle varies with load, pulse-frequency modulation, orcurrent-limited pulse-frequency modulation as those control techniquesare known to those skilled in the art.

The enabling circuit 204 may include a comparator circuit 232, an outputdecision circuit 234, and discharge path including resistors R1, R2 andswitches K3, K4. The comparator circuit 232 may be any variety ofcircuits for comparing the charge on the capacitor 206 to apredetermined reference value charge represented by control signal Vdschinput to the comparator circuit. The comparator circuit 232 isconfigured to provide an output signal to maintain the SRC 202 in thefirst state, e.g., a non-operating state or a predetermined suitableoperating state, if the charge on the capacitor 206 is higher than thereference charge. The comparator circuit 232 is also configured toprovide output signal to switch the SRC 202 from the first state to asecond state, e.g., another operating state where switches K1, K2 areunder control of the controller 220, if the charge on the capacitor 206is less than the reference charge.

The output decision circuit 234 may be any variety of circuits forperforming a desired logic function. The output decision circuit 234accepts the output signal dsch from the comparator circuit 232 and mayfurther accept a logic signal chginh from a separate source. The outputdecision circuit 234 may be configured to output an enabling signal onpath 215 to the controller 220 if signal dsch indicates the charge onthe capacitor 206 is less than a reference charge level. The outputdecision circuit 234 may alternately require signal dsch to indicate thecharge on the capacitor 206 is less than a reference charge level, andrequire signal chginh to be in a predetermined state, e.g., low state,when at least one non energy storage element condition is satisfied. Theoutput decision circuit 234 is not a necessary part of the enablingcircuit 204 if the logic signal chginh is not input to the enablingcircuit. In this case, the comparator circuit 232 would provide theenabling signal if the charge on the capacitor 206 is less than thereference charge.

The enabling circuit 204 may also include a discharge path includingresistors R1, R2, and switches K3, K4 to discharge the capacitor 206below a reference level. Switches K3, K4 may be any type of transistorsand for simplicity are drawn to represent MOS type transistors. Detailedoperation of an enabling circuit and the discharge path is made laterwith reference to the exemplary enabling circuit of FIG. 2A.

Turning to FIG. 2A, a circuit diagram of one exemplary enabling circuit204 a is illustrated. Those skilled in the art will recognize a varietyof circuit configurations which may be utilized in an enabling circuitconsistent with the present invention. In the exemplary embodiment ofFIG. 2A, the comparator circuit 232 of FIG. 2 includes a comparator 232a having its positive input terminal accepting the input signalrepresentative of the charge on the capacitor 206 and its negative inputterminal accepting the other input control signal, Vdsch, representativeof a reference value charge.

The output decision circuit 234 includes a NOR gate 234 a. The NOR gate234 a accepts the output from the comparator 232 a and the controlsignal chginh. The output of the NOR gate 234 a is HIGH only if allinputs are LOW. Otherwise, the output of the NOR gate 234 a is LOW. Inthis embodiment, the enabling signal sent by the enabling circuit 204 tothe controller 220 to enable the regulating circuit 202 to switch fromthe first state, e.g. switches K1, K2 open, to the second state state,e.g., switches K1, K2 under control of controller 220, is sent when theoutput of the NOR gate is HIGH.

Operation of the exemplary enabling circuit 204 a in conjunction withthe system 200 is described further herein with reference to the TruthTable of Table 1. Table 1 details the status of the various controlsignals and switches relative to each other with the output decisioncircuit 234 functioning as the NOR gate 234 a of FIG. 2A. The status ofcontrol signal chginh input to the enabling circuit 204, control signaldsch output from comparator 232 a, control signal chgen output from theNOR gate 234 a, switches K1, K2 of the SRC 202, and switches K3, K4 ofthe enabling circuit 204 are all detailed in Table 1. TABLE 1 chginh H HL L dsch H L H L chgen L L L H K1 OFF OFF OFF PWM K2 OFF OFF OFF PWM K3ON ON ON OFF K4 ON ON ON OFF

As illustrated and described more fully herein, the exemplary enablingcircuit 204 a advantageously does not enable switches K1, K2 of the SRC204 to be controlled by the proper control technique, e.g., PWM control,and maintains switches K1, K2 in an OFF position, until the capacitor206 of the SRC 202 is discharged below a reference charge (controlsignal dsch is L), and at least one other non capacitor charge relatedcondition for operation of the regulating circuit (control signal chginhis L) is satisfied. As such, negative voltage transients that mayotherwise occur at the output terminal 211 of the SRC 202 are avoided.

As illustrated in the first substantive column of Table 1, if the chargelevel on capacitor 206 is greater than a reference charge level asrepresented by control signal Vdsch input to the comparator 232 a, thenthe control signal dsch output from the comparator 232 a is HIGH. Theoutput control signal dsch from the comparator 232 a is then input tothe NOR gate 234 a.

The other input to the NOR gate 234 a may be the control signal chginhfrom a separate source. Such control signal chginh is representative ofat least one non capacitor related condition pertinent to operation ofthe SRC 202. In this embodiment, if this chginh signal is LOW, at leastone and perhaps all other non capacitor related conditions pertinent tooperation of the SRC 202 are satisfactory. If this chginh signal isHIGH, such condition or conditions are unsatisfactory. Accordingly, ifthe output control signal dsch from the comparator 232 a is HIGH and thechginh signal is also HIGH, the output signal from the NOR gate 234 a isLOW. Thus, switches K1 and K2 remain OFF or open and the operation ofthe SRC 202 is delayed. As such, if the SRC is operating to provide acharging current to an associated rechargeable battery, such chargingcurrent would not be provided in this instance, nor would current flowfrom the capacitor 206 through the inductor 208 be possible in thisinstance.

Turning to the second substantive column of Table 1, if the capacitor206 is discharged below the reference charge represented by controlsignal Vdsch, the output control signal dsch from the comparator 232 agoes LOW indicating the charge level on the capacitor 206 is acceptable.However, if another non capacitor charge related condition isunsatisfactory, the control signal chginh remains HIGH. As such, theoutput of the NOR gate 234 a remains LOW and the operation of the SRC202 is still delayed.

Turning to the third substantive column of Table 1, if the controlsignal chginh is LOW representing a satisfactory starting condition forat least one non-charge related condition, but the output control signaldsch from the comparator 232 a is HIGH, then the output signal chgenfrom the NOR gate 234 a remains LOW. As such, the operation of the SRC202 is still delayed.

As illustrated in the fourth substantive column of Table 1, it is notuntil the capacitor 206 is discharged below the reference charge level(control signal dsch output from the comparator 232 a is LOW), and atleast one if not all other non-charge related conditions for operationof the regulating circuit (control signal chginh is LOW) are satisfied,that the control signal chgen output of the NOR gate 234 a is HIGH. Oncethe control signal chgen is HIGH, switches K3 and K4 of the enablingcircuit 204 open or are in an OFF position. The HIGH control signalchgen enables the controller 220 to drive the SRC 202. Hence, switchesK1, K2 are controlled by an appropriate control technique, e.g., PWM.

If the enabling signal is not present in this embodiment, switches K3,K4 are closed or in an ON position. A discharge path for the capacitor206 is then created through the switches K3, K4. Switch K3 is furthercoupled in series to resistor R1, while switch K4 is further coupled inseries to resistor R2. Resistor R1 has a resistive value that is higherthan the resistive value for resistor R2. As such, when switches K3, K4are closed because no enabling signal from the NOR gate 234 a ispresent, a discharge path is created and resistor R2 serves to dischargethe capacitor 206.

Turning to FIG. 3, an exemplary battery charging system 300 generallyincluding a power source 344, an SRC 302, an enabling circuit 304consistent with the invention, a rechargeable battery 340, and a batterycharging controller 320 is illustrated. Such a battery charging system300 may be used in a variety of portable electronic devices such aslaptop computers, cell phones, pagers, personal digital assistants, andthe like to provide and control power flow to a rechargeable battery340, e.g., a lithium, nickel-cadmium, or nickel-metal hydride battery.

A sensor such as sense resistor 346 may be used in order to provide asensed signal to the controller 320 indicative of the charging currentIchg to the battery. Such a controller 320 is typically an integratedcircuit (IC) and may be sensitive to negative voltage transients thatmay otherwise occur at either terminal 349, 351 if the energy storageelement 306 is not properly discharged below a reference charge value.Such negative voltage transients may appear on either terminal 349, 351due to oscillation induced by an inductor and capacitor group of the SRC302.

Once a power source 344 is properly coupled to the system 300, itprovides an input DC voltage signal to the SRC 302. The power source 344may be an AC/DC adapter configured to receive conventional AC voltagefrom a power outlet and convert it to an applicable DC voltage, or aDC/DC adapter such as a “cigarette lighter” type adapter configured toplug into that type of socket, or other types of power sources.

Advantageously, the SRC 302 accepts power input from the power source344 and converts it to a proper output voltage and current level forproviding a charging current Ichg to the battery 340 only if thecontroller 320 receives an enabling signal from the enabling circuit 304along path 315. Otherwise, the controller 320 delays providing acharging current Ichg to the battery 340, while keeping the SRC 302 in apredetermined suitable state. This predetermined suitable state may beany variety of states as determined by the position of various switchesin the SRC 302. In the previous embodiment of FIG. 2, switches K1, K2 ofSRC 202 were chosen to be in an open state. The SRC 302 is controlled bythe controller 320 by any variety of control techniques, e.g., PWM,known by those skilled in the art.

Advantageously therefore, the enabling circuit 304 delays providing ofthe charging current Ichg to the battery 340 until the energy storageelement 306 is discharged below a reference charge. The enabling circuit304 may also further delay the charging current Ichg to the battery 340until another control signal, e.g., signal chginh, from another sourceindicates that at least one non-charge related condition pertinent tooperation of the SRC 302 is satisfactory.

The embodiments that have been described herein, however, are but someof the several which utilize this invention and are set forth here byway of illustration but not of limitation. It is obvious that many otherembodiments, which will be readily apparent to those skilled in the art,may be made without departing materially from the spirit and scope ofthe invention.

1. An enabling circuit for enabling a DC-DC converter having a capacitorcoupled to an output terminal of said DC-DC converter to be controlledby a control signal, said enabling circuit comprising: a comparisoncircuit configured to compare a feedback signal representative of acharge on said capacitor with a signal representative of a referencecharge and to provide an output in response to said comparison toprevent said DC-DC converter from being controlled by said controlsignal if said charge on said capacitor is greater than said referencecharge.
 2. The enabling circuit of claim 1, wherein said comparisoncircuit comprises a discharge path configured to discharge said chargeon said capacitor if said charge is greater than said reference charge.3. The enabling circuit of claim 2, wherein said discharge pathcomprises at least one switch, said at least one switch configured toclose to create said discharge path from said capacitor if said chargeon said capacitor is greater than said reference charge.
 4. The enablingcircuit of claim 3, further comprising an output decision circuitconfigured to receive at least said output from said comparison circuitand provide an enabling signal to enable said DC-DC converter to switchfrom a first state wherein said DC-DC converter is not controlled bysaid control signal to a second state wherein said DC-DC converter iscontrolled by said control signal if said charge on said capacitor isless than said reference charge.
 5. The enabling circuit of claim 4,wherein said output decision circuit is further configured to receive acondition signal, and wherein said output decision circuit provides saidenabling signal in response to said condition signal.
 6. The enablingcircuit of claim 4, wherein said comparison circuit comprises acomparator.
 7. The enabling circuit of claim 1 wherein said comparisoncircuit is configured to enable said DC-DC converter to be controlled bysaid control signal if said charge on said capacitor is less than saidreference charge.
 8. A system comprising: a DC-DC converter configuredto accept an input power level from a power source and provide aregulated output power level to a load, said DC-DC converter having acapacitor coupled to an output terminal of said DC-DC converter; and anenabling circuit for enabling said DC-DC converter to be controlled by acontrol signal, said enabling circuit comprising: a comparison circuitconfigured to compare a feedback signal representative of a charge onsaid capacitor with a signal representative of a reference charge and toprovide an output in response to said comparison to prevent said DC-DCconverter from being controlled by said control signal if said charge onsaid capacitor is greater than said reference charge.
 9. The system ofclaim 8, wherein said comparison circuit comprises a discharge pathconfigured to discharge said charge on said capacitor if said charge isgreater than said reference charge.
 10. The system of claim 9, whereinsaid discharge path comprises at least one switch, said at least oneswitch configured to close to create said discharge path from saidcapacitor if said charge on said capacitor is greater than saidreference charge.
 11. The system of claim 9, further comprising anoutput decision circuit configured to receive at least said output fromsaid comparison circuit and provide an enabling signal to enable saidDC-DC converter to switch from a first state wherein said DC-DCconverter is not controlled by said control signal to a second statewherein said DC-DC converter is controlled by said control signal ifsaid charge on said capacitor is less than said reference charge. 12.The enabling circuit of claim 11, wherein said output decision circuitis further configured to receive a condition signal, and wherein saidoutput decision circuit provides said enabling signal in response tosaid condition signal.
 13. The system of claim 8, wherein saidcomparison circuit comprises a comparator.
 14. The system of claim 8wherein said comparison circuit is configured to enable said DC-DCconverter to be controlled by said control signal if said charge on saidcapacitor is less than said reference charge.
 15. A method comprising:monitoring a charge on a capacitor of a DC-DC converter coupled to anoutput terminal of said DC-DC converter; preventing charging of saidcapacitor if said charge of said capacitor is greater than a referencecharge; and enabling said DC-DC converter to be controlled by a controlsignal if said charge of said capacitor is less than said referencecharge.
 16. The method of claim 15, further comprising discharging saidcapacitor if said charge is greater than said reference charge.
 17. Themethod of claim 16, wherein said discharge operation further comprisesclosing a switch to create a discharge path from said capacitor toground.
 18. The method of claim 15 wherein said enabling step occurs ifat least one non capacitor condition is satisfied.